1. Field of Invention
The present invention relates to a communication system and method; and more particularly to a combined fixed and mobile communication system and method. While the invention is subject to a wide range of applications, it is particularly suited for use with a cellular communication system.
2. Discussion of Related Art
Fixed telephone service may be provided by several different types of systems. The most common is wiring telephone lines directly to the home. However, in more primitive, or sparsely settled countries, such systems, are either not economically feasible, or the time required for installation is substantially protracted. Wireless telephone systems for fixed installations have been proposed that overcome some of the disadvantages of wired systems. One such system involves using a 450 MHz frequency to provide a connection between a local office and a subscriber's home. Such a system is designed for low density rural application, and does not achieve the spectral efficiency and frequency reuse needed to service high density city environments.
Other systems have been proposed that utilize a large number of transmission sites (microcells) which must be deployed to geographically cover a specifically inhabitated area. This multiplicity of sites must receive electrical power, and also be tied back to one or more switching centers. However, the cost of power, backhaul facilities and the microcells themselves are not always cost effective for certain applications.
Cellular communication systems are now used for mobile telephone service where radio coverage is divided into cells; and each cell is assigned a number of available radio frequencies. A mobile telephone station transmits and receives control and voice communication information from a base station within the same cell. The base stations of each cell are controlled by a cellular system switching and control network that provides connection with the world wide telecommunication system.
There are many different formats for cellular communication systems. In one type of digital cellular communication system, for example, the allocated transmit and receive frequency bands are divided into individual radio channels of communication, with the transmit and receive frequencies being separated from each other. Each radio channel has a frame format, that is, each channel transmits a succession of frames, which has a duration typically of forty milliseconds, and constitutes one cycle of a regularly recurring series. Each subscriber's information is transmitted in one of six time slots. Each time slot includes one hundred sixty-two symbols, and has a duration of approximately 6.67 milliseconds. Each slot corresponds to a burst of RF energy that includes compressed digital speech signals, which are decompressed at the receiving station and converted to analog or sixty-four Kilobits per second (KBPS) pulse code modulation (PCM) speech.
Typically, an encoder is provided for each transmitter, both at a base station and a mobile station, which compresses the speech signals before modulation and transmission thereof. One type of digital cellular communication system, for example, includes a technique for low rate speech coding, referred to as Codebook Excited Linear Prediction (CELP), which involves searching a table or codebook of randomly distributed excitation codewords for that codeword which, when filtered through pitch and linear predictive coding short term synthesis filters, produces an output sequence which is closest to the input sequence. This output sequence of synthesized speech codes occurs upon excitation of the input sequence which, in turn, occurs upon the introduction of the digital equivalent of analog speech.
Provided the antennas of each of the wireless telephones are not shielded, it is technically possible for fixed subscribers to use the mobile telephones. Due to the much greater subscriber densities in fixed systems, such use could soon result in too many subscribers for a single cell, thus, over taxing the capacity of a single cell site. Also, the expense of a complete mobile telephone for each subscriber could be a financial burden.
In light of the foregoing, there is a need for a communication system for fixed subscribers which provides quality as high as a mobile cellular communication system, but which is suitable for high density applications in a cost effective manner.